1. Field of the Invention
The present invention relates to semiconductor devices and methods. In particular, the present invention relates to integrated fuses.
2. Background Art
Integrated fuses, such as polysilicon fuses, are used as programmable elements in a variety of applications. An integrated fuse can be programmed or set to an open or closed state. The open and closed states are also referred to as conditions where the fuse is “blown” or “unblown.” Typically, an integrated fuse can be programmed to change from an unblown state to a blown state by applying an electric current of sufficient strength to increase the resistance of the fuse. Example applications which use integrated fuses as programmable elements include: programmable read only memory (PROM), static random access memory (SRAM), redundancy implementation in logic devices, die identification, electrically programmable feature selection, and CMOS logic elements. See, Alvai, M., et al., “A PROM Element Based on Salicide Agglomeration of Poly Fuses in a CMOS Logic Process,” IEDM:855–858 (1997) (referred to herein as the “Alvai article”), and Kalnitsky, A., et al., “CoSi2 Integrated Fuses on Poly Silicon for Low Voltage 0.18 μm CMOS Applications,” IEDM:765–768 (1999) (referred to herein as the “Kalnitsky article”), both of which are incorporated herein by reference in their entirety.
FIG. 1A is a top view that shows the geometry of a conventional polysilicon fuse 100. FIG. 1B is a cross-sectional view taken along line A—A of FIG. 1A. As shown in FIG. 1A, polysilicon fuse 100 generally includes two contact regions 102, 108 bridged by two transition regions 104, 106 and fuse neck 105. The center of fuse 100 is indicated by the dashed line C. Polysilicon fuse 100 is made up of a heavily doped N type (N+) or heavily doped P type (P+) polysilicon layer 110 with or without silicide layer 120 as shown in FIG. 1B. The unblown fuse resistance of fuse 100 is preferably low in the range of 50 to 100 ohms (Ω). Polysilicon fuse 100 becomes electrically open by applying a sufficient amount of energy in a form of current flow so as to blow the fuse. In this example, both silicide layer 120 and polysilicon layer 110 can be blown open as shown in FIG. 1B. The difference in the pre(unblown) and post(blown) fuse resistance values can be made many orders of magnitude such that the blown fuse acts as an open circuit. This open state is shown in FIG. 1B by the presence of a gap within fuse neck 105. This gap may not necessarily occur at the center, however, and may instead start at the contact regions.
In addition, as CMOS device sizes decrease, it is increasingly difficult to blow a polysilicon fuse since the corresponding supply voltage also becomes small. An external power supply is often needed to generate sufficient current flow to create an adequate open fuse state. This is a more costly solution. It is therefore sufficient to blow open only the silicide layer to program the polysilicon fuse. This can be done by the limited internal power supply.
FIGS. 2A and 2B show one conventional polysilicon fuse 200 used as a programmable element as described in the above-referenced Alvai article. As shown in FIG. 2A, polysilicon fuse 200 is formed from a silicide layer 220 on the top of a polysilicon layer 210. Polysilicon layer 210 can be undoped, N+ doped, or P+ doped as shown in FIG. 2A. Silicide layer 220 can include titanium silicide, nickel silicide, platinum silicide, or cobalt silicide. FIG. 2B shows fuse 200 in an open state where the silicide layer has been programmed to create a region 230 where the resistance is made higher because current is now conducted through the higher resistance polysilicon layer 210. The break in the silicide layer may not necessarily occur at the center, however, and may instead start at the contact regions.
Region 230 is created by passing electrical current through silicide layer 220 as part of an agglomeration process. The location where region 230 occurs along fuse 200 is referred to the “fusing location.” The fusing location has been reported to be a function of temperature gradient in addition to fuse geometry and pre fuse resistance. See, Alvai, M., et al., IEDM:855–858 (1997).
It is increasingly desirable to achieve a polysilicon fuse having a high mean post fuse resistance with a tight post fuse resistance distribution for a given geometry and pre fuse resistance. Agglomeration needs to reliably start at or very near the center of the fuse neck and proceed toward the contact regions. Further, an improved post fuse resistance distribution and an increased minimum resistance value in the post fuse resistance distribution is needed.